How to install the add-on to import Rhino .3dm files
Don't download the master branch, instead download the latest release here or use the following links to download directly.
Windows: https://github.com/jesterKing/import_3dm/releases/download/0.0.8/import_3dm-v0.0.8.win.zip
Macos: https://github.com/jesterKing/import_3dm/releases/download/0.0.8/import_3dm-v0.0.8.macos.zip
Then install the same way as any other addon. Hope this helps!